1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, printer and the like, particularly, it relates to braking of a running member of an optical system or the like which reads an original image.
2. Description of the Prior Art
A general electrophotographic copying machine will be explained according to FIGS. 12 and 13. In FIG. 12, a photoreceptor drum (1) which rotates at a constant velocity is made of a drum base formed with such a material as aluminum and is provided with a photoreceptor material from the selenium group on its surface. Around the drum (1) in its rotational direction, there are provided a main charger (2), an exposer (3), a light emitting diode (hereinafter, referred to as LED) (5) for partial image erasing, a developer (6), a transferrer (7), a separator (8), a cleaner (10) and a discharger (11) in succession. In the charger (2), a charge wire (2b) formed with a fine tungsten wire disposed in a U-shaped case (2a) is brought close to and along the surface of the photoreceptor drum (1) axially. A voltage of about 5-6 kV is usually applied to the charge wire (2b), thereby the surface of the photoreceptor drum (1) is charged to about 600-800 V.
LED (5) is used when part of a latent image has to be erased but is not used normally The developer (6) supplies a developer consisting of a carrier (iron filings) and toner supplied from a hopper (12) via a toner supply roller (13) constituted by a sponge roller or the like, to the surface of a developing roller (15) after the developer stirred by a stirring roller (14). The toner sticks to the surface of the photoreceptor drum (1) in response to the electrostatic latent image, thereby an actual image is formed. The toner forming the actual image is transferred to a paper fed through a resist roller (16) at the transferrer (7). The separator (8) serves to separate the transferred paper from the drum (1) by applying an AC electric field to the drum to release an electrostatic force between the drum (1) and the paper.
The transferrer (7) and separator (8) include charge wires (7b), (8b) respectively.
After the paper is separated, the drum surface is cleaned at the cleaner (10) by scraping the toner with a blade (17) made with a rubber material, and further the electric charge is removed therefrom in the following discharger (11) by receiving light from a discharge lamp (18).
Numeral (19) denotes a lighting unit including an exposure lamp (20), an oval reflector (21), an auxiliary reflector (22) and a first reflecting mirror (23), which moves toward the right in FIG. 12 at a velocity V when scanning an original (24). Numeral (25) indicates a mirror unit having second and third reflecting mirrors (26), (27), which moves toward the right in FIG. 12 at a velocity of V/2 when scanning the original (24), and serves to keep the length of the light path always constant with respect to the photoreceptor drum (1). The lighting unit (19) and mirror unit (25) constitute a running optical system (28). Numeral (29) indicates a frame of the electrophotographic copying machine, (30) is a contact glass, (31) is an original cover member and (32) is a buffer comprising a sponge and the like mounted under the original cover member (31).
Light emitted from the exposure lamp (20) is reflected at the original (24) and guided to the exposer (3) through a stationary lens (34) and a fourth stationary reflecting mirror (35), after passing a slit (33) and being reflected at right angles respectively at the first, second and third reflecting mirrors (23), (26), (27).
Numerals (36), (37), (38) denote a home position photo interrupter (hereinafter, photo interrupter is referred to as PI) for detecting the home position, a brake timing PI and a paper feed timing PI respectively , which are installed immovably in order along the running path of the running optical system (28), and respond to a shield piece (39) disposed in the lighting unit of the running optical system. The home position PI (36) presents a reference position of the running optical system (28), the brake timing PI (37) applies braking to the running optical system (28) and the paper feed timing PI (38) adjusts the timing for conveying the transferring paper from the resist roller (16) toward the photoreceptor drum (1).
FIG. 13 is a partially exploded perspective view showing the running optical system (28) and its associated portions specifically. In FIG. 13, (43) is a first moving frame having the lighting unit (19) shown in FIG. 13 and (44) is a second moving frame having the mirror unit (25), which are both engaged slidably to a rod (45) at one end. On one end of the first moving frame (43), a driving wire (46) is secured by a metal piece (47) and screws (48), (49). On one end of the second moving frame (44), there is provided a pulley (50), to which the driving wire (46) is wound by one turn. The driving wire (46) is stretched between fixed portions (51), (52) via pulleys (50), (53), (54), (55), (56). The pulley (55) is mounted to a motor shaft (not shown) and serves as the driving pulley. The pulley (54) gives tension to the driving wire (46). Shafts of the pulleys (53), (56) are fixed to the copying machine body. The first and second moving frames (43), (44) reciprocate in a direction (F) by movement of the wire. Numerals (57), (58) indicate fixing means for fixing ends of the rod (45) to the copying machine body. Rail (59) is a rail supporting the other ends of the first and second moving frames (43), (44) slidably.
Numeral (60) denotes a PI for forced return, also shown in FIG. 12, which is stationary and responds to the shield piece (39) of the optical system (28). In general, the return operation of the running optical system (28) is effected automatically when a count value of a timer counter has reached a timer value set in the resist timer responsive to the copy size selected by an operation panel of the electrophotographic copying machine, thus the PI for forced return (60) is disposed only for the sake of safety.
The running optical system (28) moves toward the right from the home position PI (36) as the starting point, and conveys the transferring paper toward the photoreceptor drum (1) from the resist roller (16) by the paper feed timing PI (38). When the count value of the timer counter has reached the timer value responsive to the copy size set through the operation panel key (not shown), the running optical system (28) is returned, and when arriving at the brake timing PI (37), the braking is applied to stop at the home position PI (36). Thereafter, the same operations are repeated.
In the aforesaid configuration, there is also such prior art that the brake timing is designated by the timer counter without installing the brake timing PI (37).
As described hereinabove, though the brake timing during the return operation in the prior art is effected by the brake timing PI output or decided by the timer counter output, positions concerning brake timing in these cases are all fixed.
However, since the braking capacity changes relatively when the weight of the running optical system varies due to its replacement, or when the moving load is changed due to stains of the rod (45) or rail (59), an accurate stop at the home position can not be achieved by fixed brake timing.
Furthermore, ordinary running operations continued forcibly when the moving load of the running optical system increases extremely due to secular or sudden stains incurred by the component members of the running path such as the rod (45) or rail (59), and this may cause damage to the running optical system and running path.